When a surface watercraft travels through the water, the hull creates a bow wave which increases in length as the speed of the watercraft increases. The length of the bow wave becomes equal to the length of the watercraft when the speed/length factor of the watercraft, that is, the factor relating speed to the square root of the water line length, reaches a generally accepted value of 1.34. To attain speeds in excess of those corresponding to a speed/length factor of 1.34, a watercraft must climb onto and over the bow wave.
The speed/length factor divides watercraft into three categories. At one extreme are displacement watercraft which have a maximum speed/length factor of about 1.34. This relatively low speed/length factor corresponds to a relatively low maximum speed. Attempting to increase this maximum speed by increasing the power output of the propulsion unit or units causes the stern to be pulled down and results in little, if any, gain in speed.
At the other extreme are planing watercraft which have a speed/length factor of about 2.5 to about 8. This type of watercraft can pull itself up onto the bow wave so that friction is reduced and speed can be increased. Top speed is nevertheless restricted by the maximum size of the propulsion unit or units which can be installed in the watercraft. Furthermore, planing watercraft are far less stable than displacement watercraft.
Between these extremes are semi-displacement watercraft having a speed/length factor of about 1.34 to about 2.5. Semi-displacement watercraft are designed in such a manner that they can begin to climb the bow wave. This makes it possible for watercraft of this type to achieve speeds which, although not as high as those of planing watercraft, exceed the maximum speed of displacement watercraft. Moreover, while the stability of semi-displacement watercraft is not as good as that of displacement watercraft, it is better than the stability of planing watercraft.
Aside from the speed restrictions imposed on surface watercraft by the fact that the hull is in contact with the surface of the water, such watercraft are greatly influenced by the condition of the water surface, e.g., by swells and waves. Additionally, surface watercraft must be designed to withstand the severe forces of the large waves generated during storms.
The drawbacks of surface watercraft are largely overcome by submersible watercraft. At depths greater than about three times the hull diameter, submersible watercraft are virtually unaffected by the conditions at the surface of the water. Furthermore, since submersible watercraft travel below the surface and do not generate a bow wave, they are free of the speed restrictions dictated by such wave. However, submersible watercraft generally are not acceptable for use as pleasure cruisers and are not well-adapted for carrying cargo. Moreover, submersible watercraft must meet the highly specialized criteria imposed by an underwater environment in which the watercraft must provide an atmosphere suitable for crew containment, comfort and life support.
Another class of watercraft attempts to combine the desirable features of surface watercraft with those of submersible watercraft. This hybrid watercraft consists of an abovewater deck which is supported by a pair of submerged, torpedo-like hulls equipped with engines. The hulls and the deck are connected to one another by vertical foils. Since neither the hulls nor the deck are in contact with the surface of the water, there is essentially no bow wave to restrict the speed of the hybrid watercraft. Moreover, inasmuch as the submerged hulls only serve for propulsion, the criteria imposed on the hulls are much less severe than those for submersible watercraft. The abovewater deck allows the hybrid watercraft to be used for pleasure. Although the hybrid watercraft offers significant advantages over surface watercraft and submersible watercraft, the abovewater deck is nevertheless exposed to the action of the waves. Especially during storms, this substantially affects passenger comfort as well as the performance of the hybrid watercraft. In addition, the hybrid watercraft must have the strength necessary to fixedly connect the hulls to the abovewater deck which rigidly spans the hulls above the surface of the water.